1. Field of the Invention
The invention relates generally to a radar system having an operator positionable variable range mark. Such a range mark is used by an operator to determine the distance from the radar zero position to the selected target upon which the range mark is positioned. More particularly, the invention relates to such a radar system which additionally employs digital processing of received radar signals.
2. Description of the Prior Art
Previous radar systems which employed a variable range ring operated primarily using analog signal processing in the PPI mode. Received radar signals were displayed at substantially the same rate at which they were received. Such systems worked reasonably well at longer ranges in which the writing rate upon the cathode-ray tube screen of the display device of the radar system was sufficiently slow to produce an acceptably high brightness level. Also, for the time periods ordinarily involved in the longer ranges, the range to a target could be determined with a generally sufficient amount of precision. However, for short ranges, the writing rate of the cathode-ray tube beam became unacceptably high so that the brightness level was reduced down to unacceptably low levels. Moreover, it became more and more difficult to accurately measure the distance to a target as the range decreased because of the short time periods involved.
Later, PPI radar systems were developed in which the incoming radar signals were digitized then processed before display. To overcome the problem of low brightness at short ranges, in one such system the digitized radar signals were read into a storage memory at a first high rate then read out subsequently at a slower rate so that the beam of the cathode-ray tube would be deflected at a slower writing rate than if the signals were displayed in real time thereby providing increased brightness at shorter radar ranges.